The technology of integrated circuits has been characterized by a continuing increase in the density with which devices can be formed in a silicon semiconductor chip or substrate. The interconnection of such high density devices within the chip requires the formation on its surface of conductors that are extremely small and spaced closely together, and in many cases conductive patterns that overlap, or, in the terminology of the technology, are vertically spaced at different conductor levels. The use of two or more levels of conductors, of course, requires a deposition of a dependable insulation on the lower level of conductors so that the overlapping upper level can be made without the risk of accidental short circuits or other conductive anomalies.
The lower-most level of conductors, usually referred to as "the first conductor level," is normally insulated from the semiconductor wafer or chip portion of the integrated circuit by silicon dioxide which is easily formed over the silicon wafer by, for example, chemical vapor deposition (CVD). Silicon dioxide is a favored material for conductor insulation because of its good thermal and electrical characteristics and because it is easily patterned by selective masking and etching. Unfortunately, CVD cannot normally be used to deposit silicon dioxide over the first conductor level because the high temperatures normally required would melt or damage conductors made of such metals as aluminum.
For this reason, it has become a widely accepted practice to deposit silicon dioxide from a radio-frequency plasma containing a silicon component. Such plasmas are formed in reactors including opposite parallel plate electrodes, one of which is grounded and one excited by a radio-frequency source. The plasma provides energy for enhancing the reaction required for silicon dioxide formation and deposition at temperatures lower than those required for CVD, and for this reason is sometimes known as plasma enhanced chemical vapor deposition or PECVD. It should be noted that other oxides such as silicon monoxide may be deposited by this process, but the predominant deposited material is silicon dioxide, which is the term that will be used herein. Also, deposition is on a "substrate," which may be part of the semiconductor, the metal conductors, or previously deposited or grown silicon dioxide.
The source of silicon used in plasma deposition may be of any of a number of silane gases or other gases containing silicon. The copending application of Dean et al., Ser. No. 175,567, filed Mar. 31, 1988, assigned to Bell Laboratories, Inc., hereby incorporated herein by reference, describes the advantages of using tetraethoxysilane (TEOS) together with oxygen as the plasma deposition atmosphere. The application describes the difficulty of making silicon dioxide depositions on integrated circuits having closely spaced metal conductors such as to provide dependable insulation for all conductors. The usual approach to this problem is to try to make a deposition as conformal as possible; that is, to make the silicon dioxide coating conform as closely as possible to the outer surface configuration of the conductors. Particular attention has been made to insure a dependable coverage of the sharp corners of conductors that inevitably result from the usual mask and etch process for defining the conductor patterns.
However, we have found that even conformal coatings create problems when it is desired to make more than one level of conductor patterns. In particular, the conformal coatings of closely-spaced adjacent conductors tend to grow together in such a way as to create a void or other characteristic imperfection within the deposited silicon dioxide. After deposition, the upper surface of a silicon dioxide is typically etched or ground to make it flat and a subsequent conductor pattern is formed over the deposited silicon dioxide. Voids or serious imperfections in the deposited silicon dioxide often cause unpredictable variations of the structural and insulative qualities of the coating.
Accordingly, there is a need for a method for depositing silicon dioxide over closely-spaced metal conductors that will provide dependable insulation and support for a second level of electrodes made on such deposited silicon dioxide.